The present invention relates generally to the treatment of flue gas for the removal of pollutants therefrom, and more particularly, to a process for limiting the buildup of chlorides in the reactant mixture.
The treatment of the flue gas generated during the combustion of fossil fuels for the removal of various pollutants, particularly sulfur oxides, contained in the flue gas is well known in the prior art. Over the years, a number of processes have been developed or proposed for removing sulfur oxides, other gaseous pollutants, ans particulate matter, such as fly ash, from the flue gas. The most common process in commercial use today is termed wet scrubbing and involves contacting the flue gas with an aqueous medium containing chemicals which will react with the pollutants present in the flue gas and thereby remove the pollutants from the flue gas.
In a typical wet scrubber for removing sulfur oxides, the flue gas is passed into a scrubber module, such as a spray tower or other absorber type known in the art, wherein the flue gas is contacted with a spray of the aqueous reactant. The sulfur oxides contained in the flue gas react with the absorbent additive in the mixture to form the bisulfite, sulfite and sulfate salts of the additive. The spent absorbent mixture, rich in sulfur salts, is allowed to drain from the scrubber module and is collected in a reaction tank to which additional unreacted sulfur oxide absorbent is added to regenerate the absorbent mixture which is then recirculated to the scrubber module and sprayed into the incoming flue gas.
In the reaction tank, the relatively water-insoluable sulfite and sulfate salts of the absorbent crystalize thereby forming a concentration of solids in the mixture. To control the solids concentration of the mixture a portion of the mixture is passed from the reaction tank through a dewatering cycle. In a typical dewatering cycle, the spent reactant mixture is passed to a thickener wherein the sulfite and sulfate salts are allowed to settle out. The sulfite and sulfate salts settle in the thickener leaving a relatively solids free supernatant liquor. The sulfite and sulfate salts, referred to as sludge, are removed from the thickener and passed to a dewatering device, typically a vacuum filter or a filter press, wherein much of the liquid remaining in the sludge is removed to yield a sludge suitable for eventual disposal. The additional liquid removed from the in the dewatering device and the supernatant liquid from the thickener are mixed with makeup water to form a recycle liquor. This recycle liquor is pumped to the reaction tank and mixed with additional sulfur oxide absorbent to form additional reactant mixture and is recycled to the scrubbing system.
A major problem associated with recycling the liquid removed from the scrubber sludge is that water soluble impurities present therein are also recycled. These impurities may enter the scrubber cycle either through their presence in the fuel from which the flue gas was generated or from their presence in the makeup water being continuously added to the slurry to replace water lost in the scrubber or in the sludge dewatering cycle. By recycling the impurities the concentration of these impurities in the reactant mixture tends to build up.
An impurity which can cause serious problems is chlorine. In the low pH environment in the scrubber, the presence of chloride ions in the reactant mixture can contribute to corrosion of the structure of the scrubber module, the piping, various tanks associated with the scrubber and other contacted surfaces. As chloride is normally present in the makeup water added to the reactant mixture and often in the fuel, particularly coal, from which the flue gas is generated, the rapid buildup of chloride ions can be a major problem.
One obvious solution to this problem is simply to discard rather than recycle the liquid removed from scrubber sludge. However, because of the presence of chlorine and other undissolved impurities, the liquid cannot be merely discharged to the environment without creating a potential risk of a serious water pollution problem. Although techniques such as reverse osmosis and vapor compression evaporation have been proposed to remove water-soluble impurities from this liquid, such techniques are expensive and exhibit questionable reliability.